Engine valves in internal combustion engines are commonly biased toward a closed position by a compressed spring. The bias force provided by the compressed spring when it is at its maximum operational, or installed, length is referred to as "preload", and designated herein by the symbol, N.sub.o. The magnitude of the preload force N.sub.o is determined by the amount of initial deflection .delta. of the spring multiplied by its compliance coefficient k. This relationship is represented mathematically by the formula: N.sub.o =k.delta.. It should be noted that the compliance coefficient k of a spring element is often interchangeably identified by several terms, such as spring rate, spring constant, or compliance rate. The preload force N.sub.o should be of a magnitude sufficient to assure that the valve head is securely maintained on its seat during the closing period. The force required to assure seating of the head at high engine speeds is higher than that required at relatively low engine speed and, therefore it is customary to compress the valve springs upon installation in an engine, by an amount sufficient to provide the higher preload force. The total bias force N imposed by the spring on the valve varies over time during the opening and closing cycle of the valve head, and is the sum of the preload N.sub.o and the additional deflection .delta. of the spring at a specific instant in the cycle, times its constant k. That is, N=k.delta.+N.sub.o, with N being a force acting in a direction along the line of movement of the valve and normal, or perpendicular to the interface between the cam or follower and the push rod or linkage surfaces associated with the valve.
The boundary friction force F at the cam/follower interface is the principal parameter determining wear on the respective components comprising the interface. The boundary friction force F is in the mixed lubrication regime and is equal to the friction coefficient .mu. times the normal load N, i.e., F=.mu.N. At the cam/follower interface, the friction coefficient .mu. is a strong function of the normal load N, such that a small reduction in the normal load significantly decreases the friction coefficient .mu.. Thus, reductions in the normal load N have a double impact on the boundary friction force F and, in that the preload N.sub.o constitutes one component of the normal load N, it is desirable to reduce the preload N.sub.o force when the engine is operating a lower speeds. Importantly, reduction in the preload N.sub.o at lower engine operating speeds increases the engine operating efficiency and reduces fuel consumption at those speeds.
Several systems have been proposed by which a variable preload is imposed on engine valves. For example, U.S. Pat. No. 4,446,825 issued May 8, 1984 to Dante S. Giardini et al describes an engine valve system in which a fixed preload is imposed on the valve spring during operation of the engine at higher speeds. The fixed preload is applied by a piston in communication with a hydraulic chamber that can be selectively pressurized or emptied through a single port in the chamber. Fluid trapped in the chamber is thus subjected to heat buildup during extended high speed operation of the engine as a result of direct heat transfer and the conversion of absorbed mechanical energy to heat. Without recirculation or other means for the transfer of heat from the fluid trapped in the hydraulic chamber, the fluid may absorb sufficient heat to raise the fluid to a temperature above its vapor point, causing concurrent increases in the chamber pressure and volume. This event is advantageously used to affect changes in the length of self-adjusting hydraulic tappets. However, in a spring preload system, the increase in chamber volume adds unintended preload forces on the spring that impose excessive, unnecessary and undesirable forces on the valve train, increasing wear and decreasing engine efficiency.
Also, Japanese published examined application 58-217711 to Kaoru Katayama discloses a valve spring system having two springs, one inside the other, acting in parallel. A preload is selectively applied on the inner spring when the engine is operating in a high speed range by a moveable hydraulic piston similar to that proposed by Giardini et at. The Katayama valve device has the same inherent problems with respect to fluid temperature build up and absence of fluid circulation fluid through the chamber controlling the position of the piston.
Another Japanese published examined application, 61-255203 to Takeo Fuwa, describes a closed system having a fluid path between two valves in which chambers controlling the position of hydraulic lifters are interconnected. Fluid within the system is confined to the two chambers and their interconnecting passageway such that a reduction in the volume of one chamber results in a corresponding increase in the volume of the other chamber. The Fuwa valve device also has inherent problems with respect to fluid temperature build up attributable to the absence of circulating fluid through the piston actuating chamber.
The present invention is directed to overcoming the problems set forth above. It is desirable to have an engine valve system in which a variable preload can be controllably imposed to reduce friction forces at critical interfaces in the valve train during low speed operation of the engine. Further, it is desirable that fluid employed in the actuation of the variable preload system be recirculatable to alleviate undesirable excessive temperature increases in the actuating fluid. It is also desirable to have such a system in which the stiffness of the valve spring varies in response to variations in the rotational speed of the engine.